The p53 gene is mutated in over 50 different types of human cancers, including familial and spontaneous cancers, and is believed to be the most commonly mutated gene in human cancer (Zambetti and Levine, FASEB (1993) 7:855-865; Hollstein, et al., Nucleic Acids Res. (1994) 22:3551-3555). Greater than 90% of mutations in the p53 gene are missense mutations that alter a single amino acid that inactivates p53 function. Aberrant forms of human p53 are associated with poor prognosis, more aggressive tumors, metastasis, and short survival rates (Mitsudomi et al., Clin Cancer Res 2000 October; 6(10):4055-63; Koshland, Science (1993) 262:1953).
The human p53 protein normally functions as a central integrator of signals including DNA damage, hypoxia, nucleotide deprivation, and oncogene activation (Prives, Cell (1998) 95:5-8). In response to these signals, p53 protein levels are greatly increased with the result that the accumulated p53 activates cell cycle arrest or apoptosis depending on the nature and strength of these signals. Indeed, multiple lines of experimental evidence have pointed to a key role for p53 as a tumor suppressor (Levine, Cell (1997) 88:323-331). For example, homozygous p53 “knockout” mice are developmentally normal but exhibit nearly 100% incidence of neoplasia in the first year of life (Donehower et al., Nature (1992) 356:215-221).
The biochemical mechanisms and pathways through which p53 functions in normal and cancerous cells are not fully understood, but one clearly important aspect of p53 function is its activity as a gene-specific transcriptional activator. Among the genes with known p53-response elements are several with well-characterized roles in either regulation of the cell cycle or apoptosis, including GADD45, p21/Waf1/Cip1, cyclin G, Bax, IGF-BP3, and MDM2 (Levine, Cell (1997) 88:323-331).
Microtubules have a central role in the regulation of cell shape and polarity during differentiation, chromosome partitioning at mitosis, and intracellular transport. Microtubules undergo rearrangements involving rapid transitions between stable and dynamic states during these processes. Microtubule affinity regulating kinases (MARK) are a novel family of protein kinases that phosphorylate microtubule-associated proteins and trigger microtubule disruption (Drewes, G., et al. (1997) Cell 89: 297-308).
Microtubule affinity regulating kinase 1 (MARK1) is a serine/threonine kinase that phosphorylates microtubule-associated protein tau, leading to disruption of microtubules. It shares 90% amino acid homology with the rat version of MARK1, and demonstrates ubiquitous expression with highest levels in testis and brain (Nagase, T. et al. (2000) DNA Res. 7:143-150).
EMK1 (MARK2) is a serine/threonine protein kinase with two isoforms, which differ by the presence or absence of a 162-bp alternative exon (Espinosa, L. and Navarro, E. (1998) Cytogenet. Cell Genet. 81:278-282). Both human isoforms are coexpressed in a number of cell lines and tissues, with the highest expression found in heart, brain, placenta, skeletal muscle, and pancreas, and at lower levels in lung, liver, and kidney (Inglis, J. et al. (1993) Mammalian Genome 4: 401-403). Due to the physical location of this gene, 11q12-q13, EMK1 is a candidate gene for carcinogenic events (Courseaux, A. et al. (1995) Mammalian Genome 6: 311-312), and has been associated with colon and prostate cancer (Moore, T. M., et al. (2000) J Biol Chem 275:4311-22; Navarro, E., et al. (1999) Biochim Biophys Acta 1450: 254-64).
Microtubule affinity regulating kinase 3 (MARK3) was originally identified as a marker (KP78) induced by treatment with DNA damaging agents. The loss of MARK3 was associated with carcinogenesis in the pancreas (Parsa, I. (1988) Cell Growth Differ. 9: 197-208). MARK3 may be involved in cell cycle regulation, and alterations in the MARK3 gene may lead to carcinogenesis. MARK 3 is ubiquitously expressed throughout human tissues, with an additional 3.0 Kb transcript present in the heart (Peng, C. et al. (1998) Cell Growth Differ. 9: 197-208).
MAP/microtubule affinity-regulating kinase like 1 (MARKL1) has two isoforms (Nagase, T. et al. (2001) DNA Res. 8: 85-95), is activated by the beta-catenin/Tcf complex in hepatic cell lines, and may be involved in hepatic carcinogenesis (Kato, T. et al. (2001). Neoplasia 3:4-9).
The ability to manipulate the genomes of model organisms such as Drosophila provides a powerful means to analyze biochemical processes that, due to significant evolutionary conservation, has direct relevance to more complex vertebrate organisms. Due to a high level of gene and pathway conservation, the strong similarity of cellular processes, and the functional conservation of genes between these model organisms and mammals, identification of the involvement of novel genes in particular pathways and their functions in such model organisms can directly contribute to the understanding of the correlative pathways and methods of modulating them in mammals (see, for example, Mechler B M et al., 1985 EMBO J 4:1551-1557; Gateff E. 1982 Adv. Cancer Res. 37: 33-74; Watson K L., et al., 1994 J Cell Sci. 18: 19-33; Miklos G L, and Rubin G M. 1996 Cell 86:521-529; Wassarman D A, et al., 1995 Curr Opin Gen Dev 5: 44-50; and Booth D R. 1999 Cancer Metastasis Rev. 18: 261-284). For example, a genetic screen can be carried out in an invertebrate model organism having underexpression (e.g. knockout) or overexpression of a gene (referred to as a “genetic entry point”) that yields a visible phenotype. Additional genes are mutated in a random or targeted manner. When a gene mutation changes the original phenotype caused by the mutation in the genetic entry point, the gene is identified as a “modifier” involved in the same or overlapping pathway as the genetic entry point. When the genetic entry point is an ortholog of a human gene implicated in a disease pathway, such as p53, modifier genes can be identified that may be attractive candidate targets for novel therapeutics.
All references cited herein, including sequence information in referenced Genbank identifier numbers and website references, are incorporated herein in their entireties.